Vehicles with internal combustion engines typically use a Lundell machine as a primary electrical power source. The Lundell machine provides a rugged and economical solution for electrical power demand in vehicles.
Referring now to FIG. 1, a vehicle electrical generator 10 with a field regulator 14 is shown. The field regulator 14 is also known as a single quadrant chopper because field current and voltage are either positive or zero at any given time. The generator 10 also includes a Lundell alternator 18 (also known as a claw pole alternator), which is a wound-field, 3-phase synchronous machine. The output of the Lundell alternator 18 communicates with a 3-phase avalanche bridge rectifier 22 to produce DC power.
The field regulator 14 generates an error signal based on a difference between a terminal voltage (across B+ and B−) and a reference Vref. The field regulator 14 includes a pulse width modulator (PWM) controller 26 that modulates (PWM) a power transistor Q1, which is in series with a field winding 30. A duty cycle of the transistor Q1 depends on a field current that is required to maintain an output voltage at a desired level for a given speed and load condition. Diode DF provides a freewheeling path for the field current when the transistor Q1 is switched off.
Referring now to FIG. 2, a test circuit 40 for evaluating transient performance of the vehicle electrical generator 10 is shown. The generator 10 is connected to battery 44 and a load 48. The battery 44 can be a 42V battery and the generator 10 can be a 42V generator, although other voltage levels are contemplated. Switches Sg, Sb, and Sl are used for disconnecting the generator 10, the battery 44 and/or the load 48.
In a full load dump, the generator 10 (supplying rated current) is suddenly disconnected from the battery 44 and most of the load 48. The full load dump produces a severe transient on the power bus as is shown in FIGS. 3A and 3B. With a normal rectifier, the peak transient voltage Vpeak can reach levels several times the nominal generator voltage Vnom. The transient duration Tload—dump may last several hundred milliseconds prior to dropping below a maximum specified generator voltage Vmax.
Avalanche rectifiers clamp the peak transient generator voltage Vpeak to acceptable levels by absorbing generator output power until a magnetic field decays to lower levels. A duration of the load dump transient Tload-dump is primarily dependent on a field time constant in a single quadrant regulator. The field current decays at its natural rate while freewheeling through the diode DF when the transistor Q1 is turned off due to over-voltage.
The load dump energy absorbed by the avalanche rectifiers in higher power generators may require multiple devices in parallel or much larger devices to assure reliable operation. These devices significantly increase the cost, size and mass of the alternator, which is undesirable for vehicle applications.